FIG. 1A is an exploded perspective view of a conventional rotary input device 2, and FIG. 1B is a sectional view taken along the line B-B in FIG. 1A. A disc-like rotary member 7 is formed of an insulator, and a user can turn smoothly the rotary member 7 with his finger 3 lightly held on the top of the disc-like rotary member 7. The conventional rotary input device 2 is capable of sensing the angle of rotation of the rotary member 7 caused by the finger 3.
In one part of a casing 4 shown in FIG. 1B, there is formed an operation region 2A sinking in a direction perpendicular to the rotary member 7 and (which direction will hereinafter be referred to as z-direction). In the operation region 2A there are formed recesses 8, for example, at eight circumferential positions, respectively. In these recesses 8 there are disposed eight electrodes 6a, 6b, 6c, . . . , 6h, respectively. On the surfaces of the electrodes 6a to 6h in the recesses 8 there is laminated an insulating sheet 5 such that the operation region 2A is covered over the entire area thereof with the insulating sheet 5.
A rotary shaft 16 is fixed to a bearing 14 formed at the center of the operation region 2A, with the rotary member 7 rotatably mounted on the rotary shaft 16. The rotary member 7 is as thin as 1 mm or less, and is formed from a relatively hard sheet of resin such, for example, as PET (polyethylene terephthalate).
The diameter of the rotary member 7 is smaller than the operation region 2A, and the rotary member 7 is adapted to be capable of turning in the operation region 2A while sliding on the surface of the insulating sheet 5. Incidentally, the frictional resistance between the insulating sheet 5 and the rotary member 7 is small, permitting the rotary member 7 to turn smoothly.
In the conventional rotary input device 2, turning the finger 3 while lightly pressing the finger 3 against the surface of the rotary member 7 as shown in FIG. 1, the rotary member 7 may be turned together with the finger 3. This enables the user to prevent the finger 3 from straying widely from the operation region 2A while turning the rotary member 7. Further, since the user can manipulate a mechanically rotatable member, he can get a feeling of actually manipulating the rotary input device 2 (a feeling of manipulation or confidence inspired by the manipulation).
Next, a description will be given of a method of detecting the angle of rotation caused by the finger 3. Bringing the finger 3 close to or into contact with the surface of the rotary member 7, the human body serves as a grounding member producing an electrostatic capacitance C between the finger 3 and any one or adjacent ones of the electrodes 6a to 6h. The capacitance C of a parallel-plate capacitor can be expressed by the equation given below.C=∈·S/d  (1)where ∈ is the dielectric constant between the electrode 6 and the finger 3, S is the area over which the electrode 6 and the finger 3 face each other, and d is the distance between the electrode 6 and the finger 3 facing each other.
Turning the finger 3 along with the rotary member 7 toward the electrode 6a, for instance, the distance d between the finger 3 and the electrode 6a decreases and the area S increases. As a result, the value of the capacitance C increases as seen from Eq. (1). On the contrary, as the finger 3 moves away from the electrode 6a while turning the rotary member, the distance d increases with a corresponding decrease in the area S. Consequently, the value of the capacitance C decreases accordingly.
Through utilization of such variations in the value of the capacitance C, the rotary input device 2 is capable of detecting the rotation angle of the rotary member. The details of the rotary input device 2 are given in Patent literature 1. The art similar to the rotary input device 2 is also described in Patent literature 2.    Patent literature 1: Japanese Patent Application Laid Open No. 2004-311196    Patent literature 2: Japanese Patent Application Laid Open No. 2005-149856